The present invention relates to data storage devices employing rotating data storage media. As described in U.S. Pat. No. 7,394,607, the location of the write head relative to the desired location of the track is the position error signal (PES), which is generated during passage of the read head over a “servo burst region” contained within a servo zone (also termed a “servo spoke”). Typical disks may have more than 250 approximately radial servo zones, thereby providing PES values roughly every 1° of rotation of the disk medium. U.S. Pat. Publication No. 2011/0188149A1 is an example of the use of servo bursts for generation of PES values.
U.S. Pat. No. 6,111,714 illustrates the use of adaptive control of the write inhibit signal as a function of the measured velocity of the write head, as may occur during data storage device operation in a operational vibration or shock environment, as may be the case for a computer playing loud audios or movies. The benefits of reducing the number of write inhibit events in such an environment are demonstrated in U.S. Pat. No. 7,633,698B2, which discusses the use of an accelerometer to detect the acceleration of the data storage device, enabling a differentiation between vibratory and one-time shock events. In particular, when the disk drive is not subject to vibration, the write inhibit threshold may be lowered to enable more sensitive detection of one-time shock events. Under vibratory conditions, the write inhibit threshold may be increased to a more normal level, thereby avoiding an excessive number of WI events. Operational shock may result in large PES values over short time periods during which it may be preferred to prevent write or read events, thereby avoiding writing outside the desired track regions, and possibly even “head crashes”.
FIG. 1 is a schematic diagram 100 of two neighboring tracks 102 and 104 in a disk storage device with write inhibit based on a fixed PES range. The track center 106 for track 102 is shown as a curved line, representing a sequence of position error signals (PES) generated each time the read head passes over a servo burst region while writing track 102. The track center 108 for track 104 differs from the track center 106 for track 102 as shown. Dashed line 110 represents the maximum positive PES value for writing track 102—any PES values larger than this (representing cases where track center 106 extends to the right of line 110), will induce a write inhibit event for track 102. The distance 112 between track center 102 and PES limit 110 is set equal to L (see FIG. 2). Similarly, the PES limits when writing track 104 are shown as dashed lines 114 and 118. Anytime track center 108 extends to the left of line 114, or to the right of line 118, a write inhibit event will be triggered. Spacings 116 and 120 are both equal to “L”. The inter-track spacing 122 is described in FIG. 2.
FIG. 2 is a schematic graph 200 of the probabilities of various PES values 250 and 252 for two neighboring tracks in a data storage device employing shingled method recording (SMR) without feed-forward (FF) as illustrated in FIG. 1. The desired center 202 of track 1 corresponds to track 102 in FIG. 1, while the desired center 204 of track 2 corresponds to track 104. The vertical axis in FIG. 2 corresponds to the probability of the PES having the values shown along the horizontal axis 270. Experimental measurements confirm that we can assume that the PES values are roughly normally-distributed, falling on the two standard deviation curves shown centered on the track centers 202 and 204. From the specifications of the recording system, a minimum allowable track center-to-center spacing, W, is determined. To ensure data integrity, a write inhibit (WI) condition is imposed whenever the PES value exceeds limits of ±L, thus the write-inhibit criteria are (on a sector-by-sector basis):PES(k)≧L, andPES(k)≦−L, where k=the track number (either 1 or 2 in this example). Each of these events (which are obviously mutually-exclusive) occurs with a probability of Q(L/σ), where the Q-function is the tail probability:Q(x)=0.5−0.5erf(x/√2),and erf is the error function. Thus the total probability of one or the other event occurring is clearly 2 Q(L/σ).
For proper data storage device performance, we require that 2 Q(L/σ)˜10−3, which determines the value for L in units of T. Since in practice L≅0.13 T, we can determine a value for σ (also in units of T) by solving for σ in this equation:2Q(L/σ)=10−3 which gives a value for the standard deviation σ≅0.04 T (the factor of “2” arises due to the possibility for both positive and negative head excursions). Thus a PES value of 0.13 T represents a deviation of the head from the desired track position of roughly 0.13/0.04=3.25 standard deviations. As shown in FIG. 2, an additional margin, E, is added to account for PES noise so that the probability of a head excursion of L+E is approximately 10−6:2Q((L+E)/σ)˜10−6.In practice, values of E of roughly 0.05 T are used. Typical values of T may be around 17 nm with track widths of 7 nm. Thus we now can determine a value for “W” in units of T, since T=W+2L+2E:W=T−2L−2E=T−2(0.13T)−2(0.05T)=0.64T. Clearly this method for determining T may be somewhat conservative, since in all cases a worst-case potential value for the PES values of neighboring tracks is used to determine T and W. As FIG. 1 illustrates, if track center 106 happens to be to the left of track 102, then the spacing between track center 108 and track 106 will exceed the minimum allowable spacing W, resulting in needlessly large spacings between the written tracks. If track center 106 is at location 260 in FIG. 2 at the same time as track center 108 is at location 262 in FIG. 2, then the tracks will be at their minimum allowable spacing of W—in essentially all other situations, the actual spacing between track centers 106 and 108 will be larger than W, indicating sub-optimal (too low) storage densities (tracks per inch or TPI).
Thus it would be desirable to account for the actual track center of a neighboring track when writing the next track, especially when employing shingled writing, in order to reduce the amount of time that the inter-track spacing exceeds the minimum allowable value of W.
There are three parameters which may be used to characterize the performance of data storage devices:
1) The number of write inhibit (WI) hits—we would like to minimize this, all other parameters being constant,
2) The number of hard errors (HE)—we would also like to minimize this, again all other parameters being constant, and
3) The data storage density (tracks per inch, TPI)—we would like to maximize this, all other parameters being constant.
For different regions of the disk medium, different overall optimization strategies may be desirable:
1) In E-regions (temporary cache regions), improving performance may be preferred, possibly at the expense of data storage density. This is because in these regions, the data will eventually be moved to an I-region, but for the moment, this data is being stored at the maximum rate possible, thus minimizing the number of WI and HE events is preferred over maximizing the TPI.2) In I-regions (final “home” or destination regions), improving the areal data storage density (TPI) may be preferred. For these regions, more dense storage is preferred since I-regions are written whenever possible (rewriting data from E-regions) or when large sequential writing is in progress.
Thus it would be advantageous to provide a method for feed forward write inhibit (FFWI) control which optimizes the writing strategy for both I- and E-regions, possibly with differing optimization strategies. It would also be advantageous to provide a method for write inhibit control which improves the performance of a disk drive in an operational vibration environment.